The low content of free water and high swelling pressure in compacted bentonite, planned to be used as a buffer in nuclear waste repositories, create adverse conditions for direct measurements of the chemical conditions. This paper presents results from on-line measurements with Eh and pH electrodes in water-saturated compacted MX-80 bentonite. The Eh was measured with Au- and Pt-wires as electrodes, while the pH was determined with IrOx electrodes. The measurements have been carried out in closed bentonite cells and in cells where the bentonite is in contact with an outside solution through a filter plate, so that the experimental conditions can be changed during the experiment. The results suggest that it is possible to measure pH and Eh in compacted bentonite, which improves the possibilities of obtaining information about the chemical processes occurring in the bentonite.